Initial progress was made towards the major objective of this study, i.e., the determination of the relationship between soluble human histocompatibility (HL-A) antigens in serum and the susceptibility to cancer. It was possible thus far to develop a simple and effective procedure for the isolation of soluble HL-A antigens from serum. Progress was made in improving the efficacy of the antigen blocking test utilized for the serological evaluation of HL-A antigens in serum by elucidating the reaction mechanisms between cell membrane, antibodies and complement during the lytic process. Thus it appeard that in some cases variability in susceptibility to lysis is due to changes in cell membrane structure or to the ability of the cell to repair complement induced damage at certain intervals during its growth cycle. It was also feasible to delineate more clearly the relationship between cell surface expression of HL-A alloantigens and other markers such as beta2- microglobulin and complement (C3b and C3d) receptors. In this regard a sensitive rosette formation and immunoadherence assay were developed for the detection of C3 receptors and a highly specific and sensitive radioimmunoassay was established for the detection of beta2- microglobulin.